Shock Cooling

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afcraig2010
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Shock Cooling

Post by afcraig2010 »

Is shock cooling modeled? Whether or not it is, what are the recommended procedures to avoid it in real life? My only experience is with smaller engines. My friend just got a Mooney so I'll be able to learn there.

Anyways, just wanted to bring this up to let other sim pilots know that this is something to be aware of. I would like to hear what other's opinions are.

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Re: Shock Cooling

Post by EnDSchultz »

Good question. That said, I have heard that rapidly growing circles are starting to consider the idea of shock cooling to be nothing but a myth. Since it's apparently such a debated topic, I wouldn't be surprised if it's not included as such. The most I would expect is that rapid cooling (such as from chopping the power and making a high-speed descent) would increase rate of cylinder wear if you get into the habit of it. Accusim is generally pretty forgiving when it comes to the likelihood of catastrophic failure, anyhow.

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Re: Shock Cooling

Post by DHenriques_ »

afcraig2010 wrote:Is shock cooling modeled? Whether or not it is, what are the recommended procedures to avoid it in real life? My only experience is with smaller engines. My friend just got a Mooney so I'll be able to learn there.

Anyways, just wanted to bring this up to let other sim pilots know that this is something to be aware of. I would like to hear what other's opinions are.
The book is still out on shock cooling :-)
Shock cooling is easily controlled in prolonged descent by maintaining a mid range power setting, watching thr temps and controlling the airspeed with pitch.
The general rule in all propeller aircraft is don't idle an engine in a descent from altitude to avoid overspeed. Plan well enough ahead that you can maintain a medium airspeed and power range descent profile.
Like all things pertaining to flying, good flying habits and good planning avoid most issues.
Dudley Henriques

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Scott - A2A
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Re: Shock Cooling

Post by Scott - A2A »

I will echo what Dudley says here. In Accu-Sim we do model rapid temperature change metal fatigue in the cylinders that weakens the walls, but it's very mild as we all know of flight school airplanes that last well beyond TBO with their throttles being pulled several times a day.

Personally, I am a nut in terms of avoiding shock cooling. It's just my nature and job to think this way. But to speak to what Dudley is saying, it could mostly be in my head and not really hurting the cylinders much. However, I don't take any chances and always make sure the engine has plenty of time to slowly cool down. If Lycoming engines were $4,000 instead of $40,000, maybe I wouldn't be so concerned. Either way, this is how I avoid shock cooling on descent:

When towards the end of a cruise I slowly pull the throttle back, sometimes just 1-2" at a time and let the plane gradually lose altitude. The cylinders will start to cool, maybe 10 deg per minute or so. When the cylinder head temps have come down 40 or 50 degrees, then I start to establish a desired descent. It's really that buffer between cruise and descent that I pay close attention to the initial cooling.

Scott.
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AKar
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Re: Shock Cooling

Post by AKar »

Can't wait to see how many good topics 'yet another Cessna' brings up! :)

See here for one view on shock cooling. Scroll down to Descents and the OWT of "Shock Cooling", though whole article is good read. The writer, John Deakin, seems very informed about these engines, and has done his homework thoroughly. (Though there are just some bits and pieces in his vast amount of writing up there that I find hard to agree, or disagree even - and note that some of the finer bits there might not / do not apply generally, and to the Accusim planes for example.)

Shock cooling is one of those topics that is more of a debate of religion than of science. Some folks go so far that they use their mental limits of that many inches back in power in that amount of time even during ground runs (!!!) to avoid shock cooling - of course a misinformed practice. Others don't buy that its an issue on some engines at all because, as Scott points out, many planes in flight training use even have had their engines extended beyond the TBO hours while treated without much of regard to shock cooling.

I'd do just about what Scott and Dudley advised. And I personally feel the way Scott says it's been modeled in Accusim is very good - I don't like that issues are exaggerated just because they exist (speaking of those instant destruction sim plane engines). One good advice in real life is to check the maintenance history and see how 'lucky' that individual airplane has been with its cylinders. If you find that there has been a cylinder replaced in just about every 50-hour visit, then I'd start to reconsider the way of operating the plane! 'Unlucky' engines are sometimes explained by the way they are operated, though some engines just are worse than the others with no obvious reasons.


-Esa

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Re: Shock Cooling

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I totally agree

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afcraig2010
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Re: Shock Cooling

Post by afcraig2010 »

Esa,

I've read some of those AvWeb articles and I read the shock cooling one a while back. I've got good instructors that believe it's real and then I hear stories where the engines are just fine, like all the examples he gives. It seems to be a hard OWT to crack. I want to really learn about how systems work and learn why the old wives tails are just rules of thumb that the instructor has passed along.

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AKar
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Re: Shock Cooling

Post by AKar »

afcraig2010,

I believe the shock cooling, as in real life, shows some classic signs of an issue that has not a simple linear causality traceable to much of anything. Therefore, the debate misses the ball when it gets into whether it exists as an issue or not.

Of course, quick temperature changes do cause stress to the cylinders. The actual stress depends as much on how evenly and how quickly the cylinder is cooled - in turn depending on the actual installation; the position, the engine, the airplane and how it's operated. How the cylinder handles the stress depends on the condition of the cylinder but also on the quality and actual metallurgy of its material - the QC on airplane parts generally suck, so the variations can be rather extreme here.

This is why I strongly believe that we tend to have 'lucky' and 'unlucky' airplanes and engines. It shows especially well with cylinder issues: some glider towers or training airplanes take something one could consider extreme abuse from shock cooling point of view. They handle it for years after years, for thousands of cycles. There the stress caused seems to be well within margins. In that kind of extreme use, perhaps the possible bad parts get changed very quickly because they simply cannot withstand the use. Note that the manufacturer's recommended CHTs are quite high, in high-cycle use I believe those who say they are unacceptably high. This quick replacement interval happens until the bad parts are changed to those that are particularly good ones, if not straight from the engine shop, then after very few first hundreds of hours when they give up and are replaced, and after that we've got a magically good engine that seems to take whatever we can throw to it.

What further complicates the analysis of this issue is that the high and rough use airplanes tend to have their possibly rather major maintenance events done quite routinely. For a low-cycle, leisurely flying white collar pilot it may come as a surprise that his cylinder is busted, and he needs to get that replaced right away. It gets a much more attention than one busted cylinder that is quickly swapped over on a gilder tower.


I believe that the issue is real, but whether it is significant or not, and how quickly it might become so, depends on many factors that usually are beyond of what the pilot can do.
afcraig2010 wrote:I want to really learn about how systems work
This is very good attitude, I believe! And it will give any pilot, engineer or technician a journey that involves quite many frustrations of 're-learning' stuff, but also joys of truly understanding topics that one might have not even know about a day ago. :) I've never regretted digging just a bit deeper, and I've only scratched the surface on the field of aviation.

-Esa

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afcraig2010
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Re: Shock Cooling

Post by afcraig2010 »

Yea that all makes sense. When there is no clear cut answer, we are left with an overarching rule of thumb. And it's a good point to bring up the frequency of the maintenance. At TBO, the engine may seem fine, but after how many cylinder changes? All good points. Do you think shock cooling affects the smaller engines (160hp 172, 65 hp Cub) in a negligible way?

-Craig

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AKar
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Re: Shock Cooling

Post by AKar »

Craig,
afcraig2010 wrote:And it's a good point to bring up the frequency of the maintenance. At TBO, the engine may seem fine, but after how many cylinder changes?
One important aspect we miss in general aviation at the operator level is any sort of proper reliability data. When we move up the food chain, the reliability of different components is generally monitored against fleet averages. In GA world, we're often just balancing on opinions and experiences that concern individual airplanes mostly. It makes the process of forming any opinions in case-by-case basis very ungrateful business. Often, more experienced guy you talk with, less answers he gives. Which is unfortunate, but shows just that when you get to know more, you actually just uncover the lack of knowledge and answers.

This leads to, well, how I put it earlier: often the folks around aviation just talk about 'unlucky' or bad airplanes, and of good ones. No one truly knows what sets them apart.
afcraig2010 wrote:Do you think shock cooling affects the smaller engines (160hp 172, 65 hp Cub) in a negligible way?
In general, no. I wouldn't say that the stresses a cylinder in IO-360-L2A (172R&S) meets are significantly different from those in IO-540-AB1A5 (182T) for example. The compression ratio is the same, and so is effectively the power per cylinder. Actually, the cooling in the bigger engine, the IO-540, can be somewhat more finely controlled, as there are the cowl flaps. There apparently are engine models that are known to crack their cylinders more often than others, but unfortunately I don't know if anyone has ever truly analyzed (using several samples from different operators) how those cracks are initiated and grown to critical.

We would need that knowledge to truly understand many issues that we now only know to have a loose correlation with certain kinds of engine damage.

-Esa

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Re: Shock Cooling

Post by joe bob »

I think some of these rules of thumb are hold overs from high performance aircraft. What may be an issue for a piston engine airliner descending from 30,000 feet sub zero temps may be at most trivial for a 182 going from 4000 feet to 1000 feet.
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